This invention relates to a substantially continuous method of making an article suitable for an optical waveguide preform, and more particularly to a method of forming an optical waveguide preform while continuously removing the starting member from the preform body while it is being formed.
High capacity communication systems operating around 10.sup.15 Hz are needed accomodate future increases in communication traffic. The systems are referred to as optical communication systems since 10.sup.15 Hz is within the frequency spectrum of light. Optical waveguides, which are the most promising medium for transmission at such frequencies, normally consist of an optical filament having a transparent core surrounded by transparent cladding having a refractive index which is lower than that of the core. A very thorough and complete discussion concerning operational theories of optical waveguides is contained in U.S. Pat. No. 3,157,726 issued to Hicks et al., and in the publication "Cylindrical Dielectric Waveguide Modes" by E. Snitzer, Journal of the Optical Society of America, Vol. 51, No. 5, pp. 491-498, May 1961. Other excellent sources of information concerning optical waveguides is "Fiber Optics--Principles and Applications" by N. S. Kapany, Academic Press, 1967, as well as "Fundamentals of Optical Fiber Communications" edited by Michael K. Barnoski, Academic Press, 1967.
Generally optical waveguides are made by either outside or inside vapor phase oxidation as hereinafter more particularly defined. In the inside vapor phase oxidation process a starting member or mandrel in the form of a tube is used and a coating of predetermined desired glass is deposited on the inside thereof. The structure so formed is thereafter heated, the inner hole collapsed, and drawn into an elongated filament which comprises the optical waveguide. The inner deposited coating of such a process comprises the core of the waveguide while the tubular starting member comprises the cladding. In the outside vapor phase oxidation process, a starting rod or tube is used as a starting member, bait, or mandrel, on the outside surface of which is deposited one or more layers of suitable material. Ordinarily, the starting member or mandrel is removed leaving the deposited structure as the preform for the subsequent waveguide. Where a tube is employed as a starting member and is either left in place or removed a center hole exists in the same manner as if a solid rod was removed from the deposited material. The center hole must thereafter be closed before or during drawing and a solid elongated waveguide filament drawn. In the outside process, either the starting rod forms the core of the ultimate waveguide or the first deposited layers form the core of the ultimate waveguide, while the latter deposited layers form the cladding thereof.
When a starting member or mandrel is used and is intended to remain with the structure to ultimately form the core of the waveguide, substantial exterior surface preparation such as cleaning, smoothing, or the like is necessary before deposition thereto may take place. In addition, if the starting member or mandrel is intended to remain and become the core of the waveguide, it must be of very high purity glass, and have optical properties equivalent to the quality of the waveguide core, for example, low signal attenuation and equivalent index of refraction. Whenever a starting member or mandrel is used, whether it remains part of the optical waveguide preform or is removed, much processing is required such as mandrel cleaning, removal, etching and the like.